Conductor Optimisation for Overhead Transmission Lines

Inaugural IEEE PES 2005 Conference and Exposition in AfricaDurban, South Africa, 11-15 July 2005

Conductor Optimisation for Overhead Transmission Lines

Dipeen Dama, Dzevad Muftic and Riaz VajethEskom Enterprises, Capital Expansion Department, Trans-Africa Projects

P.O.Box 6583, Halfway House, Midrand, 1685, South AfricaPhone: +27-11-205 9456, Fax: +27-11-205 9442, Email: [email protected]

Abstract – Continuous changes in the cost of suitable (conductive)material for bare overhead conductors, changes in electrical andmechanical requirements, and improvements in manufacturingtechnology, have resulted in the development of a variety ofpossible applications or options for overhead transmission lines.In the early days, simple copper wire or copper based bareconductors were used but nowadays, more cost effective solutions,such as aluminium and variations of aluminium alloy conductorsare used extensively in the power system.

The conductor of an overhead power line is considered as themost important component of the overhead line since its functionis to transfer electric power, and its contribution towards the totalcost of the line is significant. The conductor costs (material andinstallation costs) associated with the capital investment of a newoverhead power line can contribute up to 40% of the total capitalcosts of the line. Furthermore, power losses in the lines accountfor the bulk of the transmission system losses, which in SouthAfrica is about 1200MW at peak load. These are critical economicfactors which need careful analysis when selecting a conductor fora new overhead line, which will be in operation for an excess of 25years. Choosing a larger conductor configuration will have higherup front capital costs, but this may lead to lower overall life cyclecost. Consequently, much attention has to be given to the carefulselection of a conductor configuration to meet the present andpredicted future load requirements. A process needs to be followedto optimally choose a conductor and tower configuration.

This paper presents a procedure which has been formulatedand tested to optimise the selection of the conductor and towerconfiguration from an overall system point of view. The paper willalso highlight the significance of incorporating planning and loadforecast considerations, power quality constraints, voltage collapsestudies, corona and audible noise, induction and transpositionstudies, line performance studies, and life cycle cost ofmaintenance for the different options, in the optimisationalgorithm. The methodology and results of an actual case studyare presented to demonstrate the effectiveness of the proposedprocedure. The paper will provide a valuable guide to assist withthe selection of conductor and tower configurations for newoverhead transmission lines.

I. INTRODUCTION

The conductor costs associated with the capital investmentof a new overhead transmission line accounts forapproximately a third of the total capital costs.Consequently, much attention was given to the carefulselection of a conductor configuration to meet the present andpredicted future load requirements.

A new overhead line will form part of an interconnectedtransmission system and hence will affect power flowthrough the integrated network. Hence, it is extremelyimportant to consider and evaluate different conductor andtower configurations from an overall system point of view. Ifa conductor is not optimally selected, this would result inunnecessary system losses and poor voltage regulation.Choosing a larger conductor configuration will have higherup front capital costs, but this may lead to lower overall lifecycle cost. Furthermore, the choice of a conductor affects theinsulation, hardware, structures, foundations and towers for aspecific line. Thus, a process needs to be followed tooptimally choose a conductor and tower configuration.

In this paper, the authors have developed a procedurewhich has been successfully tested on numerous overheadtransmission line projects and has resulted in savings in theinvestment of the capital costs.

II. CONDUCTOR OPTIMISATION PROCESS

The following 16 step procedure has been systematicallyformulated and tested to optimise the selection of a conductorand tower configuration for an overhead transmission linefrom an overall system point of view. The first 6 steps of theoptimisation process evaluate the different possibleconfigurations from a line life cycle costing point of view.After analysing the line life cycle costing results, a fewoptions are selected for detail system analysis in steps 7 to 16.

Step 1: Obtain the Planning Requirements such as the linelength, voltage and expected reliability levels of the newoverhead power line. The designer must also consider theloading requirements for the life cycle of the line, based onthe planner’s load forecast and any step load changes duringthe life cycle of the line. The need for an emergency ratingand its duration also need to be accounted for. Possibleapplications of optical fibre ground wire (OPGW) need to bedetermined. The generation pattern, scaling of generationand new generation points also need to be taken into accountfor the system studies.

Step 2: Determine the field effect (EMF) and corona limits(radio interference and audible noise) for the conductor andtower configurations that can meet the above planningrequirements. The intended use of power line carriers, and

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the effect of the line design, especially transposition towers,need to be considered.

Step 3: Ascertain any design limitations from anenvironmental perspective. For example, there could be aservitude problem for different tower types when running thenew line parallel to another existing line, with towersadjacent to each other.

Step 4: Select a combination of options, each with asuitable conductor bundle and tower type, based on the lineloading and corona and field effect limits, and thereafterestimate the capital investment cost (CIC) for each option.The capital cost estimates must take into account changeswhich can occur after the line profiles and soil conditions areobtained in the detail design phase.

Step 5: Calculate the electrical line parameters (resistance(R), reactance (X) and susceptance (B)) for each option inboth actual and per unit (p.u.) values. These parameters arerequired for modelling purposes using a suitable simulationpackage.

Step 6: Based on the line loading forecast, calculate the totalcost of line losses (TCO) for each option for the entire lifecycle of the line. Thereafter perform a line life cycle costinganalysis (LCC) for the different options (i.e., LCC = CIC +TCO).

The line life cycle cost of a transmission line consists of theinitial capital investment plus the cost of line losses over thelife cycle of the line. It is necessary to calculate over the lifeof the line, the present value of the line losses, using the netdiscount rate prescribed by the utility’s financial guidelines.From a line life cycle costing point of view, the mosteconomic conductor configuration will be that which givesthe minimum cost in the following equation:

Capex + ��====

25

1n(1+d)-n x [cost of line losses in year ‘n’] (1)

where:d - net discount rate.n - year under consideration until the end of the

economic life of the line (say, 25 years).

Step 7: Select three to five most suitable conductor andtower options for detail system analysis studies.

Step 8: For long overhead power lines (>150km) or shorterbut highly loaded lines, perform unbalance studies todetermine the need for phase transposition or phaseoptimisation for parallel lines.

Step 9: The new overhead power line will be part of aninterconnected transmission system. Hence, it is alsoessential to consider the losses from an overall system pointof view. Thus, in order to make a better decision regardingconductor selection, detailed system analysis studies shouldbe performed in order to calculate the cost of system lossesfor each option for the entire life cycle of the line. It isnecessary to calculate over the life of the line, the presentvalue of the savings in system losses (minus value), using thenet discount rate prescribed.

Step 10: For short lines, the thermal limits need to beconsidered. Thermal capacity will be directly proportional tothe size of the conductor, or cross-sectional area (mm2).

Depending on operational conditions, thermal limits can bedecisive in the selection of a conductor. For example, ifoperation in emergency conditions requires a power transfersignificantly above the normal operation, then the selection ofthe configuration will be dictated by thermal limits ratherthan by the economical loading. Permanent operation close toor at thermal limits will always be significantly aboveeconomical loading, because of very high losses amounting,in total, to much more than the cost of construction.

Step 11: Calculate the impact on fault levels for the variousoptions. This could be an important factor for a weak part ofthe network.

Step 12: One of the main purposes of the new line is toextend the power transfer margin. The power transfer marginis dependent on the voltage collapse limits, which are relatedto the surge impedance of the line. Surge impedance loading(SIL) is a measure of the power that can be transferredwithout reactive compensation. SIL depends on theconductor bundle and line configuration.

Different sizes of conductor and bundle will give differentSIL and the price that must be paid for a possible increase ofSIL must be evaluated. There is a cost benefit for having ahigher voltage collapse or power transfer margin, as this willallow more load to be added on to the network without theneed for further reinforcement. This implies that options witha larger margin will be beneficial. It is difficult to place anexact monetary value on this benefit, but for the purposes ofthis discussion, the cost of adding shunt capacitor banks tomake up for the lost power transfer margin can be calculatedto compare the different cases.

Thus, the benefits of higher surge impedance loading need tobe verified by performing voltage analysis or voltagecollapse studies. This will allow for the benefits of a highersurge impedance loading to be quantified, which is relevantfor long overhead power lines which are voltage rather thanthermally constrained.

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Fig. I. Graphical description of the purpose of voltage collapsestudies.

Step 13: Consider the operating and maintenance costs andthe need for live-line maintenance for each conductor andtower configuration.

Step 14: Consider the need for high reliability and thedifferences in reliability for each option.

Step 15: Consider the need for flexibility in terms of ease touprate in the future and the cost and outage times for suchuprating. This could include retensioning, real timemonitoring, voltage upgrade or addition of another conductorin the bundle to change, for example, from triple tern to quadtern.

Step 16: Incorporate all the factors discussed above into anappropriate technology index to determine the optimumoption, based on the cost/benefit or weighting of each of theabove factors.

The above procedure has been summarized in the flowchartbelow.

Fig. II. 16 Step conductor optimisation process.

III. CASE STUDY

The following is an example of an actual conductoroptimisation exercise which was carried out for a newoverhead power line in South Africa’s integrated powersystem network. The names of substations have been keptconfidential to protect the end customers. The 400 kV linewas to be constructed between Substation A and SubstationD. The following highlights the process which was followedto optimise the conductor and tower selections.

Step 1: The following inputs were provided by SystemPlanners:• The line length was estimated at 96 km and the

operating voltage was 400 kV.• It is extremely difficult to predict future line loading due

to the fact that there is a lot of uncertainty in any loadforecast exercise. However, based on historicaldemand, and based on high short term load growth,growth rates for specific substations in the area ofinvestigation were estimated.

• Planning also envisaged changes in the generation andnetwork patterns during the life cycle of the line. Thesechanges were considered during the system studies.

Based on the anticipated load growth in the area ofinvestigation, and the anticipated network changes, thefollowing line loading (shown in Figure III) was predictedover a 25 year planning horizon for the A-D 400 kV line (allsimulations were performed using the software PSS/E).

Forecasted Line Loading Data

700

800

900

1000

1100

1200

1300

1400

2005

2007

2009

2011

2013

2015

2017

2019

2021

2023

2025

2027

2029

Year

MV

A

Normal Loading Emergency Loading

Fig. III. Forecasted line loading data.

In the year 2013, a new 400 kV line is anticipated and hencea drop in the line loadings when compared to the year 2012.

Steps 2, 3 and 4: Based on the above line loadingrequirements, field effect and corona limits, andenvironmental limitations, the conductor and towerconfigurations shown in Table I were compiled as a list ofworkable solutions. Field effect and corona are important

0.7

0.75

0.80

0.85

0.90

0.95

1.00

1.05

10 20 30 40 50Power Transfer (MW)

Voltage (p.u)

Add suitable size capacitor bank to match power transfer of the black option

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Rating @ 60 Degrees CMVA MVA

Conductor Tower R X B R X B Normal Emergency4 X Pelican 524 A 0.0018 0.0153 0.6959 0.0197 0.0626 0.4039 1693.3 2175.53 X Kingbird 524 A 0.0018 0.0167 0.6369 0.0197 0.0640 0.3834 1533.9 1984.9

3 X Tern (570mm) 524 A 0.0015 0.0159 0.6688 0.0194 0.0633 0.3947 1629.5 2059.82 X Bersfort 524 A 0.0013 0.0179 0.5961 0.0192 0.0652 0.3682 1550.5 1992.62 X IEC 800 524 A 0.0012 0.0180 0.5943 0.0191 0.0653 0.3675 1635.1 2251.73 X Bersfort 524 A 0.0009 0.0156 0.6846 0.0188 0.0629 0.4001 2325.8 2988.8

4 X Tern 524 A 0.0011 0.0150 0.7082 0.0190 0.0624 0.4081 2172.7 2746.33 X IEC 800 524 A 0.0008 0.0156 0.6830 0.0187 0.0629 0.3996 2452.6 3377.54 X Bersfort 524 A 0.0007 0.0141 0.7581 0.0186 0.0614 0.4240 3101.1 3985.1

Per Unit Pos Sequence Per Unit Zero Sequence

factors, which need to be carefully considered for 400 kVoperation at high altitudes.

Table I. Suitable conductor and tower configurations.

(Note: CRS refers to the flat phase configuration; standardcross rope suspension tower 524A).

All the conductors selected above are of the ACSR (lesssteel) type since they have proved to be the most effective inour practice. Other possible options, such as AAAC orACAR, can be considered in broader optimisation exercises.

Step 5: Using a suitable software package (AlternateTransients Program (ATP)), the electrical line parameters(resistance (R), reactance (X), and susceptance (B)) of thedifferent conductor and tower configurations have beencalculated as shown in Table II below.

Table II. Line parameters for conductor and tower combinations.

All the parameters were calculated on a 100 MVA base forthe entire length of the line (i.e. 96 km). A templatingtemperature of 60°C was used to cover cases of high ambienttemperature and moderate loading. It also allows for moreflexibility for future upgrades.

Steps 6 and 7: The next step is to perform a line life cyclecosting analysis for each assumed configuration. Asmentioned earlier, the line life cycle cost of an overheadpower line consists of the initial capital investment plus thecost of line losses over the life cycle of the line.

The capital investment cost (CIC) or initial cost ofconstruction for the various cases is shown in Table III. A

conservative approach to the estimates was used in order totake into account changes which can occur after the lineprofiles and soil conditions are obtained in the detailed designphase.

Table III. Capital costs (in millions of rands).

Case Configuration Tower TypesTotal(Rm)

1 4 X Pelican 524 A R 95.432 3 x Kingbird 524 A R 93.163 3 X Tern (570mm) 524 A R 107.514 2 X Bersfort 524 A R 115.905 2 X IEC 800 524 A R 120.266 3 X Bersfort 524 A R 134.857 4 X Tern 524 A R 130.568 3 X IEC 800 524 A R 148.179 4 X Bersfort 524 A R 162.56

Based on the line loading forecasts, the impedances of thedifferent conductor and tower configurations, and the cost ofgeneration (i.e. R/kWh), the total cost of line losses (TCO)was estimated for each combination over the life cycle of theline. The life cycle costing (LCC) for each option wasthereafter calculated (LCC = CIC +TCO). The table belowsummarises the results of the life cycle costing analysis.

Table IV. Results of line life cycle costing.

Case ConfigurationCIC

(Rmil)TCO

(Rmil)LCC

(Rmil)

1 4 X Pelican 95.43 122.15 217.582 3 x Kingbird 93.16 122.55 215.713 3 X Tern (570mm) 107.51 99.32 206.834 2 X Bersfort 115.9 89.31 205.215 2 X IEC 800 120.26 78.1 198.366 3 X Bersfort 134.85 60.07 194.927 4 X Tern 130.56 74.89 205.458 3 X IEC 800 148.17 52.87 201.049 4 X Bersfort 162.56 45.66 208.22

A graphical description of the above results are shown below.

Fig. IV. Graphical analysis of line life cycle costing results.

As can be seen from the above figure, for the average loadingof the line (i.e., 884 MVA), the life cycle costing curvesindicate that the 3 x Bersfort option is the cheapest and hence

Case ConfigurationOverall / Core

diameter of individualconductor (mm)

Tower

1 4 x PELICAN 20.70/4.14 CRS2 3 x KINGBIRD 23.88/4.78 CRS3 3 x TERN 27.00/6.75 CRS4 2 x BERSFORT 35.56/9.96 CRS5 2 x IEC800 37.6/7.52 CRS6 3 x BERSFORT 35.56/9.96 CRS7 4 x TERN 27.00/6.75 CRS8 3 x IEC800 37.6/7.52 CRS9 4 x BERSFORT 35.56/9.96 CRS

Conductor Optimisation: A-D 400 KV LINE

0.00

100.00

200.00

300.00

400.00

500.00

600.00

0 120 240 360 480 600 720 840 960 1080 1200 1320 1440 1560 1680

Pmean (MVA)

LC

C(R

mil)

2 BERSFORT

2 IEC800

3 BERSFORT

3 IEC800

3 KINGBIRD

3 TERN

4 BERSFORT

4 PELICAN

4 TERN

780(min)

1293 (max)

884 (Ave)

Zoomed

413


is ranked first. This option is followed closely by 2 xIEC800/3 x IEC800 options, then by 2 x Bersfort/4 x Tern/3x Tern options, then by 4 x Bersfort, and lastly by 4 xPelican. Based on these results, the configurations shown inthe table below were filtered through for detailed systemanalysis.

Table V. Options selected for detailed system analysis.Case Configuration Tower

3 3 x TERN (570mm) CRS6 3 x BERSFORT CRS7 4 X TERN CRS8 3 X IEC800 CRS9 4 x BERSFORT CRS

Step 8: The effects of voltage unbalance can be detrimental toequipment such as induction motors, power electronicconverters, and adjustable speed drives (ASDs); hence it isextremely important to perform unbalance studies todetermine the extent of voltage unbalance that would exist ona line (NRS048-2 limit is 2% for voltage unbalance in atransmission network); and to determine the best phasesequencing and transposition swap sequence in order tominimize the voltage unbalance.

The unbalance investigations revealed that a 2.54% voltageunbalance was expected at the end of the A-D 400kV line ifno transposition was considered. This value exceeded thelimit specified in the NRS048-2 power quality standard.Hence 2 transposition points were thereafter considered toreduce the voltage unbalance to within acceptable limits. Theinvestigations showed that the voltage unbalance was reducedto approximately 0.09% having implemented twotranspositions at the following locations:- Transposition point 1 – 37km from Apollo Substation; and- Transposition point 2 – 74km from Apollo Substation.

Fig. V. Results of transposition investigations.

Step 9: The A-D 400 kV line will be part of an interconnectedtransmission system and hence will affect the power flow(magnitude and direction) in the network. Hence, it isimportant to consider the losses from an overall system pointof view. Thus, in order to make a better decision regardingconductor selection, PSS/E was used to perform detailedsystem analysis studies. The results of these studies areshown in Table VI below.

Table VI. Results of system losses studies.

Case ConfigurationCapitalCost

(Rmil)

Life Cycle Benefit ofreduced MW losses

(Rmil)

Capital Cost minusBenefit from

Reduced Losses(Rmil)

3 3 X Tern (570mm) R 107.51 R 561.20 -R 453.69

6 3 X Bersfort R 134.85 R 607.70 -R 472.85

7 4 X Tern R 130.56 R 595.41 -R 464.85

8 3 X IEC 800 R 148.17 R 616.04 -R 467.87

9 4 X Bersfort R 162.56 R 628.43 -R 465.87

From the above table, it can be seen (from a life cycle benefitpoint of view) that the 4 x Bersfort configuration offers thebest life cycle benefit of reduced system losses. Over the 25year life cycle of the line, approximately R628m will besaved using this configuration. The next best MW lossesreduction is achieved with the 3 X IEC 800 configuration(R616m) and this is followed closely by the 3 x Bersfortconfiguration (R607m). After taking into account the capitalinvestment costs for each option, the most suitableconfiguration to use is the 3 x Bersfort option. This option isfollowed closely by 3 x IEC 800, 4 x Bersfort and the 4 xTern configurations (in that order).

Steps 10 and 11: Detail system analysis showed that theloading of the line throughout its 25 year life cycle is withinthe thermal limits for each configuration. More importantly,the differences in fault level at the end of the line are similarfor each configuration and hence no comparative monetarybenefit was attached to each option.

Step 12: As mentioned previously, the main purpose of thenew line is to extend the power transfer margin and this isdependent on the voltage collapse limits of each option.PSS/E was used to perform the voltage collapse studies forthe different options. The capacitor bank sizes for thedifferent options were thereafter calculated in order to matchthe power transfer capabilities of the different options. Thecost of adding the shunt capacitor banks in order to make upthe lost power transfer were thereafter estimated. The resultsof these studies have been captured in Table VII below.

TRANSPOSITION POINT 2

Tower 524A

Tower 524A

Tower 524A

Tower 520B

Tower 520B

25km

10km

37km

19km

18km

TRANSPOSITION POINT 1

Dinaledi 400

Apollo 400

DIg

SIL

EN

T

Red

White

Blue

Phase Voltages: (L-G)

Red - 206.615 kV

White - 206.341 kV

Blue - 206.387 kV

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Table VII. Results of voltage collapse studies.

Case Configuration TowerCapitalCost

(Rmil)

Max. PowerTransfer forworst N-1

(MW)

RelativePower

Transfer(MW)

Max. PowerTransfer (with 100MVAR Cap bank)

for worst N-1(MW)

Cap Bank Size (inMVAR) required to

match powertransfer of 4 x

Bersfort

Cost to makeup lost

transfer withshunts(Rm)

3 3 X Tern (570mm) 524 A 107.51 1391.066 -70.96 1469.348 90.64 8.16

6 3 X Bersfort 524 A 134.85 1428.991 -33.03 1495.890 49.38 4.80

7 4 X Tern 524 A 130.56 1436.085 -25.94 1498.883 41.31 4.02

8 3 X IEC 800 524 A 148.17 1419.027 -43.00 1494.395 57.05 5.55

9 4 X Bersfort 524 A 162.56 1462.024 N/A N/A N/A N/A

Notes:1. The main motivation for this new line is to extend the power transfer capability.2. The voltage collapse or power transfer margin was calculated using PSS/E3. TAP changers and switched shunts were locked for the purpose of this study.4. Voltage collapse point was taken as 10% before actual collapse.5. Assumed an 88 kV cap bank installed at Substation D for additional power transfer studies with cap banks.6. The following cap bank costs were assumed for the purposes of this investigation.

Size (MVAR) COST (Rm) Rm/MVAR16 4 0.2536 5 0.1472 7 0.10

100 9 0.09

It is clear from the above tabulated results that the optimumpower transfer capability under the worst N-1 contingencywas achieved with the 4 x Bersfort configuration. The nextbest power transfer capability was achieved with the 4 x Ternconfiguration; and this was followed closely by the 3 xBersfort option. The cost to add shunt capacitor banks to theother cases to match the power transfer capability of the 4 xBersfort configuration are also shown in the above table.

Step 13: It is obvious that different conductor and towerconfigurations will yield different maintenance costs, sincesome configurations require more complex line hardware.The maintenance costs of the newer tower configurations areexpected to be higher, since the personnel are not familiarwith the fastest, safest procedures to carry out themaintenance. Furthermore, performance statistics haveshown that the chances of the thinner conductors tangling andcreating vibration related problems is higher than the thickerconductors.

Research is currently being carried out to formulate amethodology for determining the operating and maintenancecosts of the different conductor and tower configurations.For the purposes of this study, the maintenance costs weredisregarded in the optimisation process. However, it isimperative that the maintenance costs be quantified andconsidered for future conductor optimisation studies.

Step 14: The planning requirement was that the A-D 400 kVline be built to the same or better reliability levels than thebetter performing nearby line (Q-R 400 kV line). The A-D400 kV transmission line will be constructed in the same areaas the Q-R 400 kV line. Thus, the new line will be subjectedto similar environmental conditions. Thus, a statisticalanalysis of the existing line can be used to define the new lineperformance expectations.

The different conductor and tower configurations areassumed to have a similar performance (from a fire, lightningand insulator pollution related faults point of view).However, future research needs to carried out in order toformulate a methodology for assigning factors to the

performance of the different conductor and towerconfigurations such that the reliability levels and performanceof the different options can be distinguished; and thereafterattach a monetary benefit to better reliability and performancelevels.

Step 15: The ease by which the line can be upgraded (interms of current only) needs to be determined. The towersselected for the various cases under study all have mechanicallimitations. However, in a case where a line is built usingtowers designed for the 4 x tern configuration and strung with3 x tern to cater for low loading scenarios, then it is possiblein the future to add another tern conductor to the 3 x ternbundle to create a 4 x tern bundle to cater for future higherloading situations. This process may, however, take a fewmonths of construction time. It is these limiting factors(outage times, construction time) which may stronglyinfluence the final decision of the optimisation process.

Step 16: From a line life cycle costing point of view, 3 xBersfort was the best option to choose for this particular line.2 x IEC800 was the next best option, but the line life cyclecosting analysis shows that it was R3.44m more expensivethan the 3 x Bersfort option.

From a system life cycle costing point of view (which onlyincludes capital costs and benefit of reduced system losses), 3x Bersfort was the best solution. This configuration is closelyfollowed by the 3 x IEC 800 option which ranked second byapproximately R5m when compared to 3 x Bersfort.

The voltage collapse studies show that the best powertransfer capability for the worst N-1 contingency wasachieved with 4 x Bersfort. The next best power transfercapability is achieved with the 4 x Tern configuration, andthis is followed closely by the 3 x Bersfort option.

All the factors discussed above have been ranked in TableVIII below.

Table VIII. Overall ranking table.

Case Configuration TowerInitial TotalCapital Cost

(Rm)

Life Cyclebenefit ofreducedsystemLosses

(Rm)

Cost to makeup lower PowerTransfer margin

with Shunts(Rm)

OverallRank(Rm)

Overall RankNormalised

(Rm)Position

3 3 X Tern (570mm) 524 A 107.51 -561.20 8.16 -445.54 22.51 5th

6 3 X Bersfort 524 A 134.85 -607.70 4.80 -468.05 0.00 1st

7 4 X Tern 524 A 130.56 -595.41 4.02 -460.83 7.21 4th

8 3 X IEC 800 524 A 148.17 -616.04 5.55 -462.32 5.73 3rd

9 4 X Bersfort 524 A 162.56 -628.43 N/A -465.87 2.18 2nd

The above conductor optimisation results indicate that the 3 xBersfort option is the cheapest. This option is followedclosely by the 4 x Bersfort, 3 x IEC 800 and 4 x Ternconfigurations in that order. Hence, based on the abovesystem analysis results, it was decided that the 3 x Bersfort(570 mm sub conductor spacing) configuration be used forthe A-D 400 kV line. The 570 mm sub-conductor spacing

415


was chosen over the 450 mm sub-conductor spacing since abigger surge impedance loading, and hence power transfermargin, can be achieved with a bigger sub-conductor spacing.

As can be seen, the design of conductors for EHVAC areoptimised with regard to the capital investments costs for theline and the operation costs for the losses. For EHVAC lines,the resistive losses determine the conductor cross section.

For HVDC lines, the selection of conductor cross sectionwith regard to the resistive losses should be done in a similarway as with EHVAC. When comparing HVDC and EHVAClines with regard to power losses, the main difference is thatcorona losses of HVDC lines are much less sensitive tovariations in weather conditions.

IV. CONCLUSION

A variety of possible options of bare conductors forapplication on overhead power lines gives the designer theopportunity to optimise the design solution in terms of cost,functionality and performance. As was shown, there is nostraight forward answer to the question of optimal conductorselection for specific operating requirements in a specificenvironment. The methodology outlined in this paper can beused as a guide for further conductor optimisation casestudies.

ACKNOWLEDGMENT

The authors wish to thank IEEE for accepting the paper aspart of the conference proceedings and all colleagues whoprovided technical advice and support.

REFERENCES

[1] “The Planning, Design and Construction of Overhead Power Lines”,Eskom Power Series, Vol. 1, Chapter 15, February 2005.

[2] R. Vajeth and D. Dama, “Conductor Optimisation for OverheadTransmission Lines”, Proceedings of IEEE Africon Conference,Gaborone, September 2004.

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Conductor Optimisation for Overhead Transmission Lines

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