DETERMINATION OF CONDUCTOR RATINGS IN ESKOM

ESKOM COPYRIGHT PROTECTED

B Morrison / December 2010 / Rev 0

STANDARD

Document Classification: Controlled Disclosure Title: Distribution Standard Part 6:

DETERMINATION OF CONDUCTOR RATINGS IN ESKOM

Unique Identifier: 32-319 Document Type: DST Revision: 0 Published date: DECEMBER 2010 Total pages: 27 Review date: DECEMBER 2015

COMPILED BY APPROVED BY FUNCTIONAL RESP AUTHORISED BY

_ _ _ _ _ _ _ _

B BRANFIELD

_ _ _ _ _ _ _ _

R STEPHEN

_ _ _ _ _ _ _ _

V SINGH

_ _ _ _ _ _ _ _

MN BAILEY Snr Consultant GM Capital Prog. for TESCOD CMDT for MD (Dx)

DATE: . DATE: . DATE: . DATE: .

Content

Page Foreword .......................................................................................................................................................... 2Introduction ...................................................................................................................................................... 21 Scope ..................................................................................................................................................... 32 Normative references............................................................................................................................. 43 Definitions and abbreviations ................................................................................................................. 4

3.1 Definitions .................................................................................................................................... 43.2 Abbreviations ............................................................................................................................... 5

4 Requirements ......................................................................................................................................... 54.1 General ........................................................................................................................................ 54.2 Probabilistic Methods ................................................................................................................... 64.3 Application of the absolute method in ESKOM ............................................................................ 7

Annex A Rate A,B and C for Eskom Overhead Conductors ...................................................................... 11Annex B Example of Time Based Rating RATE C ................................................................................... 16Annex C Definitions of Probabilities for P(CT) ........................................................................................... 21Annex D Impact Assessment ..................................................................................................................... 24

DOCUMENT CLASSIFICATION: CONTROLLED DISCLOSURE


Unique Identifier: 32-319 Type: DST Revision: 0 Page: 2 of 27


When downloaded from the IARC WEB, this document is uncontrolled and the responsibility rests with the user to ensure it is in line with the authorised version on the WEB.

Foreword

The changes made is this document align it in all aspects with the revised Transmission document, EST32-319 -DETERMINATION OF CONDUCTOR CURRENT RATINGS IN ESKOM. The author has only transcribed the information from that document, the technical content is the work of the development team.

Revision history This revision cancels and replaces revision no 1 of document no. ESKASABK1

Date Rev. Clause Remarks Dec 2010 0 - Compiled By: RA Branfield

- Original Dist doc changed to comply with Transmission document

Authorisation

This document has been seen and accepted by: Name Designation

Nick Bailey Corporate Manager Divisional Technology Rob Stephen ESKOM Distribution Capital Manager Prince Moyo Engineering Manager

This document shall apply throughout Eskom Holdings Limited, its divisions, subsidiaries and entities wherein Eskom has a controlling interest.

Development team

AA Burger Chief Engineer (EED - Trans-Africa Projects)

Dr D Muftic Transmission Line Specialist ((EED - Trans-Africa Projects))

RG Stephen Eskom Distribution Capital Manager

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 affects the safety of a transmission line as well as the types of accidents or factors that are pertinent to the utility.

In the past, Eskom used what is referred to as the Deterministic method for calculation of the conductor thermal rating or ampacity by using conservative ambient conditions. These conservative ambient conditions of 40C 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 75C and 90C. The lines were then templated at 50C according to an internal Eskom directive, EED 15/6/1-1 1970. This means that if the conductor temperature reached 50C, 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 75C, 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 50C and determine the current rating for 75C and 90C. 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 todays economic environment it is necessary to use assets more efficiently 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.

The existing practice of applying probabilistic conductor ratings served Eskom from the 1990s until 2008, when this latest conductor rating standard was completed in order to update the probabilistic ratings with the latest improvements available.

This document provides the means to calculate the line ratings at different templating temperatures. 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 utilised 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.

Keywords

Ampacity, Conductor, Temperature

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 editions indicated were valid. All 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, Title 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 Technicon 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).

Probabilistic conductor ratings revised for use in Eskom, AA Burger, Dr D Muftic, Mr RG Stephen, August 2008 (Eskom Research Report issued by Trans-Africa Projects. Parties using this document shall apply the most recent edition of the documents.

3 Definitions and abbreviations

3.1 Definitions

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.

Rate A: Maximum operating current under normal conditions. Previously know as 75C rating. Risk of exceedence (Conductor temp. > templating temp.) 9.83%

Rate B: Maximum operating current under contingency conditions. Previously know as the 90C rating. Not limited in time period. Risk of exceedence 49.11%

Rate C: Ultimate maximum operating current under emergency conditions preceding load shedding. Maximum 15 minute time period only. Function of thermal inertia of conductor. Not previously defined or utilised.

Exceedence: The time when the conductor operating temperature is greater than the design temperature.






3.2 Abbreviations

SWER: Single Wire Earth Return

CIGRE: International Council on Large Electric Systems

4 Requirements

4.1 General

The formulas used in the standard ESKASABK1 for 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. Since 2003, a more accurate model for the determination of the AC resistance of helically stranded conductors was developed using international research done by VT Morgan. At the same time, the magnetic losses as a result of using steel cores are now accurately modelled as a function of the magnetic properties of the steel core. The lay ratio and rotation of the layers of the helically stranded conductors are also part of the modelling. The introduction of mixed convection heat loss as opposed to using a combination of forced and mixed convection led to improvements in the accuracy of the calculation of convective heat loss in the low wind speed range.

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.

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

The Research to date has primarily being 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.






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. Should the surge occur 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) used to be 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 was that it assumed 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 is now solved by determining P(CT) from a large sample of actual recordings of ambient temperature, wind speed and wind direction taken at the same time The SA Weather Service do not record solar data at a large number of stations, and global solar radiation was therefore calculated theoretically as a function of the day and time of the day for the locations from where weather data was sourced.

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.

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

where

P(CT) 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 from 6 weather stations in South Africa. The weather stations were selected to avoid airport data since research findings indicated that airport weather data represent cooler temperatures and higher wind speeds. The data totals 77 years of hourly data sets, from which a random set of data was selected to calculate about 1500 values of P(CT) for each conductor considered at 50, 60, 70 and 80oC respectively.

All wind speed data below 1m/sec was modified by using a transfer function derived from parallel measurements at the same location using the 3D ultrasonic anemometer alongside the propeller anemometer used by the SA Weather Services. In this manner, the inaccurate response of the propeller anemometer could be rectified to avoid conservative values for P(CT) being calculated.






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: 1) with high traffic density road crossings 2) 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 4,65m 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(CT)) 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. 75C conductor thermal rating and 50C 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 4,2m;

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.

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 75C electrical rating, but the line template temperature was at 50C. 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 50C for normal operation (Rate A) was then calculated at 9.83%. 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 Rate B ratings in that the probability which is 5 times higher at 49.11%, was calculated for the present emergency conditions and kept constant for the different templating temperatures. Additional to the Rate A and Rate B ratings, which are both not constrained by the amount of time these ratings can be applied, a third rating, called Rate C, which is limited to a maximum period of 15 minutes of application, was introduced. Heavy conductors have significantly increased ratings because of higher mass and thermal inertia.

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 analysis, 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 without in depth investigation on the annealing of the conductor and condition of joints.

The benchmark template temperature value of 50oC will be used unless it can be clearly demonstrated that 60oC or higher values such as 70oC or 80oC must be used to prevent unjustified additional capital expense.






Annex A Rate A,B and C for Eskom Overhead Conductors (informative)

Magpie is used on SWER systems where volt drop and AC Resistance is not crucial, but longer spans are required. Hence, magpie has an unusual construction which cannot be modelled with the MathCAD program in its present form.

*For this reason, the old ratings for Normal and Emergency is directly transferred to Rate A and Rate B.

3/4/2.12 Magpie

Figure15: Construction of Magpie Conductor

33 47 58 67

40 52 62 70






Annex A (continued)

Rates A=Normal, B=Emergency, C- Short Time Current Rating






Annex A (continued)







Annex B Example of Time Based Rating RATE C INTRODUCTION

Some of the 2xDinosaur lines in the Western Grid may have to be operated for short periods of time at current levels above the emergency rating. This document describes how a 15 minute Rate C was derived for application on the lines at a current level of 60% above the 50 deg C rating for Dinosaur conductor (1842Ampere).

NEW RATINGS CALCULATED USING NEW SET OF RSA WEATHER DATA AND

IMPROVED CONDUCTOR MODELLING

The statistical ratings for Dinosaur conductor at 50 deg C for the whole of South Africa is as follows, with associated risk levels as indicated:

New Eskom ratings for Dinosaur conductor using countrywide weather data

Rating category Ampere rating at 50 deg C Statistical risk associated

(exceedence %)

Rate A 872A 3.32%

Rate B 1173A 16.61%

Rate C 15 minutes 1563A 24.92%

Rate C was derived assuming only a maximum of 15 minute time period for the conductor temperature to rise from Theta 1 to Theta (see figure below). The value of Theta corresponds to the steady state rating value associated with the 24.92% risk level (50% higher risk than the Rate B risk of 16.61%). The value of Thetam is then estimated using the thermal time constant for Dinosaur conductor. The weather parameters for the deterministic rating method were used to ensure that worst case circumstances are catered for.






Annex B (continued)

Deterministic weather data parameters

Wind speed 0.5 m/sec

Wind direction 90 degrees to conductor

Solar radiation 1120 W/sq m

Ambient temperature 40 deg C

The temperatures associated with the Rate A, B and steady state value corresponding to

Rate C 15 minutes are as follows:

Rate Temperature

Rate A Temp = 69.2 deg C *

Rate B Theta1= 85.7 deg C **

Rate C 15 minutes Steady state Theta = 95.3 deg C

* - should correspond approximately with old Eskom 75 deg C

** - should correspond approximately with old Eskom 90 deg C

The adiabatic curves, which include the mass of steel and mass of aluminium as part of the thermal inertia of the Dinosaur conductor is now presented. The calculations were done in MathCAD, and this programme was developed as part of the re-calculation of the Eskom rating tables which is presently underway.

1.

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Annex B (continued)

Temp (3600) = 110.38 deg C after 1 hour

Temp (10800) = 118.15 deg C after 3 hours

Temp (18000) = 118.65 deg C after 5 hours

The calculations above show that after an indefinite time, the conductor will reach 118.7 deg C if a current value of 1563A is maintained constantly while the weather parameters remain constant. Therefore, the 15minute rating is set at this level.

Eskom requested that a current level of 60% above the Emergency rating of 1151A (from existing tables) be checked this implies a level of 1842A, which implies that Theta = 151 deg C. It is then easy to show that a time period of 529 seconds (8.8 minutes) will be required for the conductor to reach the Theta temperature level of 95.3 deg C.

USE OF CAPE TOWN WEATHER DATA

The weather data in the Western Cape contains less severe extremes in terms of temperature, and has a considerable amount of high wind speeds, and if only this data is used, one can expect more favourable conductor ratings than when using data from the whole of RSA.






Annex B (continued)

New Eskom ratings for Dinosaur conductor using Western Cape weather data

Rating category Ampere rating at 50 deg C Statistical risk associated

(exceedence %)

Rate A 995A 3.32%

Rate B 1278A 16.61%

Rate C 15 minutes 2013A 24.92%

The temperatures associated with the Rate A, B and steady state value corresponding to

Rate C - 15 minutes are as follows:

Rate Temperature

Rate A Temp = 75.2 deg C *

Rate B Theta1= 85.7 deg C **

Rate C 15 minutes Steady state Theta = 104.2 deg C

* - should correspond approximately with old Eskom 75 deg C

** - should correspond approximately with old Eskom 90 deg C

The value of Theta is then calculated to be 173.7 deg C, which corresponds to 2013 Ampere when assuming the deterministic set of weather data.






Annex B (continued)

Temp (3600S) = 139.42C after 1hr

Temp = 168.51C after 3 hours

Temp = 172.93 deg C after 5 hours

When applying the required current of 1842Ampere, the temperature of 151C will be reached in 18.4 minutes. Therefore, there is some margin in the current level if the Cape Town 15 minute rating is used, or a longer time can be tolerated if the 1842 Ampere is strictly limited.

CONCLUSION:

If the weather data from the whole of South Africa is used, it implies that the 15 minute rating of 1563 Ampere for single Dinosaur conductor at 50C templating is less than the required current level of 1842 Ampere. Therefore, a reduced time of 8.8 minutes must be adhered to if applying the rating countrywide.

However, if the advantage of using the cooler and generally more favourable Cape Town weather date is used, the 15 minute rating goes up to 2013 Ampere. In this case, the 1842 Ampere load can actually be tolerated for up to 18.4 minutes. This applies only to Dinosaur conductor at 50C rating in Cape Town.






Annex C Definitions of Probabilities for P(CT) (informative)

The following calculation were taken from the thesis of Johan Swan, and no changes were made to the assumptions of probabilities.

Object under a line POBJ

60800= vehicle per minute on Ben Schoeman

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Surge Magnitudes PSM






Annex C (continued)

A surge magnitude of 2 p.u. will be used

95% set to is limits at thermal whilelevel age Umax voltbe willline ay that probabilit The95.0max

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1=IP The probability that the line will at some or other tie reach maximum thermal loading is set to 100%

Now the total probability of a failure can be compiled since all individual components, which are considered to be mutually independent, are defined. The total risk can be equal to the koeberg licence agreement to find the required risk of exceedence of templte conductor temperature PCT

6101 =ACCP Linked to Koeberg

025.02 =puP 310994.6 =OBJP

Switching Impulse on line PSI

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The value of Pacc represents the overall risk of a clearance infringement between a conductor of a line and ground as a function of all of the individual risk components described above.

Koebergs Nuclear Licence agreement is set at 1x10-6. In the second revision, the value of Pacc was increased by 20% to 1.2 in 1 million, and hence, the individual risk factors as described above will result in PCT = 9.83% (0.0983).






Annex C (continued)

PCT = 9.83% THIS VALUE IS APPLICABLE TO NORMAL OPERATING CONDITIONS

REFERRED TO AS RATE A

Tests conducted using 147 years of hourly data from 12 weather stations in South Africa indictated that at least 7000 weather data sets be used to calculate current values that are possible while operating the conductor at the design template temperature (e.g. 50 oC, 60 oC ...)

For emergency ratings, a factor of 5 times the exceedence level is applied to the level for normal ratings

PIemergency = 5 PCT Risk level for Rate B IS 5 TIMES HIGHER THAN RATE A

PIemergency = 49.11% Risk level for rate B

RateC = 1.63 PIemergency Risk level for Rate C - 15 minute time based rating, 1.63 times above Rate B

RateC = 80.05%






Annex D Impact Assessment (Normative)

Impact assessment form to be completed for all documents.

1 Guidelines

o All comments must be completed.

o Motivate why items are N/A (not applicable)

o Indicate actions to be taken, persons or organisations responsible for actions and deadline for action.

o Change control committees to discuss the impact assessment, and if necessary give feedback to the compiler of any omissions or errors.

2 Critical points

2.1 Importance of this document. E.g. is implementation required due to safety deficiencies, statutory requirements, technology changes, document revisions, improved service quality, improved service performance, optimised costs.

Comment: This document affects the Planning and Network Optimisation groups in so much as new tables have been introduced that impact on the current rating and operation of overhead lines.

2.2 If the document to be released impacts on statutory or legal compliance - this need to be very clearly stated and so highlighted.

Comment: The status quo remains the same in terms of legal compliance

2.3 Impact on stock holding and depletion of existing stock prior to switch over.

Comment: N/A

2.4 When will new stock be available?

Comment: N/A

2.5 Has the interchangeability of the product or item been verified - i.e. when it fails is a straight swop possible with a competitor's product?

Comment: N/A

2.6 Identify and provide details of other critical (items required for the successful implementation of this document) points to be considered in the implementation of this document.

Comment: N/A

2.7 Provide details of any comments made by the Regions regarding the implementation of this document.

Comment: (N/A during commenting phase)






Annex D (continued)

3 Implementation timeframe

3.1 Time period for implementation of requirements.

Comment: As soon as it is approved

3.2 Deadline for changeover to new item and personnel to be informed of DX wide change-over.

Comment: N/A

4 Buyers Guide and Power Office

4.1 Does the Buyers Guide or Buyers List need updating?

Comment: N/A

4.2 What Buyers Guides or items have been created?

Comment: N/A

4.3 List all assembly drawing changes that have been revised in conjunction with this document.

Comment: N/A

4.4 If the implementation of this document requires assessment by CAP, provide details under 5

4.5 Which Power Office packages have been created, modified or removed?

Comment: N/A

5 CAP / LAP Pre-Qualification Process related impacts

5.1 Is an ad-hoc re-evaluation of all currently accepted suppliers required as a result of implementation of this document?

Comment: N/A

5.2 If NO, provide motivation for issuing this specification before Acceptance Cycle Expiry date.

Comment: N/A

5.3 Are ALL suppliers (currently accepted per LAP), aware of the nature of changes contained in this document?

Comment: N/A






Annex D (continued)

5.4 Is implementation of the provisions of this document required during the current supplier qualification period?

Comment: N/A

5.5 If Yes to 5.4, what date has been set for all currently accepted suppliers to comply fully?

Comment: N/A

5.6 If Yes to 5.4, have all currently accepted suppliers been sent a prior formal notification informing them of Eskoms expectations, including the implementation date deadline?

Comment: N/A

5.7 Can the changes made, potentially impact upon the purchase price of the material/equipment?

Comment: N/A

5.8 Material group(s) affected by specification: (Refer to Pre-Qualification invitation schedule for list of material groups)

Comment: N/A

6 Training or communication

6.1 Is training required?

Comment: Yes (If NO then 6.2 6.6 will be N/A)

6.2 State the level of training required to implement this document. (E.g. awareness training, practical / on job, module, etc.)

Comment: Awareness training Planning and Network Ops

6.3 State designations of personnel that will require training.

Comment: Planning strategy to be compiled by relevant Study Committees

6.4 Is the training material available? Identify person responsible for the development of training material.

Comment: Yes Rob Stephen has presentation






Annex D (continued)

6.5 If applicable, provide details of training that will take place. (E.G. sponsor, costs, trainer, schedule of training, course material availability, training in erection / use of new equipment, maintenance training, etc).

Comment: N/a

6.6 Was Technical Training Section consulted w.r.t module development process?

Comment: No

6.7 State communications channels to be used to inform target audience.

Comment: Study Committees

7 Special tools, equipment, software

7.1 What special tools, equipment, software, etc will need to be purchased by the Region to effectively implement?

Comment: N/a

7.2 Are there stock numbers available for the new equipment?

Comment: N/A

7.3 What will be the costs of these special tools, equipment, software?

8 Finances

8.1 What total costs would the Regions be required to incur in implementing this document? Identify all cost activities associated with implementation, e.g. labour, training, tooling, stock, obsolescence

Comment:

.

.

.

Impact assessment completed by:

Name: ______R Branfield___________________________________________________________

Designation: __Snr Consultant___________________________________________________________

Documents

DETERMINATION OF CONDUCTOR RATINGS IN ESKOM