This document provides an overview of guidelines for overhead line design, including both AC and DC line design. For AC line design, it discusses electrical characteristics such as resistance, inductance and capacitance and how to optimize them. It also covers mechanical considerations, thermal rating, planning requirements and objective indicators for determining the best design. For DC line design, it discusses electrical characteristics like resistance and power transfer, as well as corona power loss. The document provides information on a variety of factors to consider for optimizing overall overhead line design.

1. GUIDE TO OVERALL
OVERHEAD LINE DESIGN

2. • AC LINE DESIGN
• ELECTRICAL CHARACTERISTICS
• MECHANICAL CHARACTERISTICS
• THERMAL RATING
• PLANNERS REQUIREMENTS
• OBJECTIVE INDICATOR
• DC LINE DESIGN
• ELECTRICAL CHARACTERISTICS
• OBJECTIVE INDICATOR
• COST OF LINES
• EXAMPLES OF OPTIMISATION
Table of contents

3. GUIDE TO OVERALL
OVERHEAD LINE
DESIGN
GUIDE TO OVERHEAD OHL DESIGN TB 638
AC LINE DESIGN

4. AC LINE DESIGN
GUIDE TO OVERHEAD OHL DESIGN TB 638
ELECTRICAL CHARACTERISTICS

5. What is a power line?
• A device to transmit power over distances.
• Design of the line can be tailor made to meet planner’s requirements.
• Load flow depends on R, X and B values.
5

6. LINE MODEL
• Reduce R and L (resistance and inductance)
• Increase C (capacitance)
6

7. Maximise Power Transfer
• Zs is surge impedance
• SIL is the surge impedance loading
• Reduce L and increase C to maximise transfer
L is series
inductance
C is shunt
capacitance
7
VLL is phase to phase
voltage

8. Determination of R, X and B
8
• Resistance is a function of
− Conductor construction material and line length
o Lay ratio, ACSR,AAAC, number of layers, diameter of strands.
− Temperature
o The higher the temperature the higher the resistance
− Current and frequency
o Transformer effect
o Eddy currents.

9. Variation with current
Penguin(1+6)
Tcond=80°C
1.04
1.02
1.00
1.14
1.12
1.10
1.08
1.06
0 100 200 300 400 500 600 700 800
I(A)
Rac/Rdc
Falcon(19+54)
Tcond=80°C
1.02
1.00
1.10
1.08
1.06
1.04
0 1000 2000 3000 4000 5000 6000 7000 8000 I(A)
Rac/Rd
c
107 mm2
9
800 mm2

10. Determination of L
10
• L is a function of Geometric mean radius (GMR) and Geometric mean Distance
(GMD)
• Larger bundle radius and closer phase spacing gives lower L

11. Determination of C
11
• To increase capacitance
keep phases closer together.

12. Summary
12
• SIL (L and C) can be varied by
− varying phase spacing closer is better
− Increasing bundle size larger is better
• Resistance can be improved by
− Varying lay ratios per layer (not practical)
− Different materials
− Homogeneous conductors

13. Corona limitations
13
• Corona can produce audible noise under certain weather conditions. This is very
difficult to mitigate. It is desirable to avoid corona inception.
− Smaller bundle radius will reduce corona up to a point.
− Wider phase spacing better
− More sub conductor bundles better.

14. AC LINE DESIGN
MECHANICAL CHARACTERISTICS
GUIDE TO OVERHEAD OHL DESIGN TB 638

15. Mechanical considerations
• Wind load is major consideration in tower design
− Less conductors in the bundle the better
− Less ultimate tensile strength (UTS) the better (Lighter strain towers). Higher
tension to increase height.
• Vibration is a function of tension
− Need to design to the recommended T/m ratio
− Small bundle sizes (twin triple), need more care in vibration damping design
• Galloping mitigation needs to be taken into account
− Includes phase configuration
− Pendulum dampers, interphase spacers
15

16. Tower top Geometry
• Tower top geometry design applies to conventional towers with metal surrounded
center phase (tower window) as well as three phases in the same window as is
the case with the cross rope suspension.
• The interaction between the phases as well as the shielding angle for the
conductors needs to be carefully designed to ensure optimal insulation co-
ordination providing the required level of reliability.

17. AC LINE DESIGN
THERMAL RATING
GUIDE TO OVERHEAD OHL DESIGN TB 638

18. Thermal loading
• Load at which the safety or annealing criteria of the line is met
− Current at which the height above the ground is in line with regulation
− Height determined by voltage and flashover distance
• Heat Balance equation used.
18

19. Joule and magnetic heating
• Joule dependent on AC resistance and temperature
• Magnetic heating dependent on current and conductor layers.
19

20. Solar heating
• Darkness of conductor
• Diameter of conductor
• Solar radiation
20

21. Convective cooling
21
• Dependent on
− the conductor diameter (bigger is better)
− Wind speed
− Temperature difference (bigger is better)
− Roughness

22. Templating temperature
22
• Templating temperature is the conductor temperature at which the height above
ground is in accordance with regulation
Conductor
Templating
temperature deg C
Normal
Amps
Emergency
Amps
TERN 50 611 814
TERN 60 784 991
TERN 70 911 1138
TERN 80 1023 1257
ZEBRA 50 642 859
ZEBRA 60 818 1049
ZEBRA 70 963 1203
ZEBRA 80 1080 1325

23. SUMMARY
SIL Corona Mechanical
loading
Thermal
rating
Phase spacing
decrease
Good Bad Good Neutral
Large al
area/cond (less
conductors)
Bad Bad Good Bad
Diameter
Bundle
increase
Good Bad Bad Neutral
High steel
content
Neutral Neutral Bad Good
23

24. AC LINE DESIGN
PLANNING REQUIREMENTS
GUIDE TO OVERHEAD OHL DESIGN TB 638

25. Planning requirements
25
• Planners need to specify the following
− Load transfer requirements
− Load profile daily, annual
− Impedance parameters, high and low
− Line Voltage for AC
− Length of line
− Location, start and end points
− Reliability requirements

26. Conductor size and temp
26

27. Conductor type
27
TACSR (Thermal resistant)
XTACSIR (Thermal resistant with
invar core)
GTACSR (Thermal resistant
aluminium with gapped core)
ACSS (Steel supported)

28. Selection of conductor
28

29. INSULATOR SELECTION
• Location of conductor bundle determined to meet insulation co-ordination
requirements
• Insulator creepage, dry arching distance, basic insulation level (BIL) determined.
− Depends on pollution levels in the line route
• Insulator configuration depends on tower selection, I or V or other.
• Material depends on pollution, vandalism, maintenance.
• Hardware depends on material (corona rings for composite), live line
requirements
• For cross rope towers I string permits less pollution accretion especially from
birds.

30. LIGHTNING CONSIDERATIONS
• Shield angle determination depends on tower type.
− Negative shield angles generally give better performance.
• Tower footing resistance needs to be determined and reduced on the line
− “crows foot”, buried earthwire, bentonite mix
• Note towers with large footprints generally give lower tower footing resistances.
− Cross rope suspensions provide excellent results
• If tower footing resistance still high may consider line surge arresters installed on
certain towers.
• Shield or earth wires are now often OPGW.
− Care to be taken for fault current in the earth wire.

31. AC LINE DESIGN
OBJECTIVE INDICATOR
GUIDE TO OVERHEAD OHL DESIGN TB 638

32. Indicator to determine best design
32
• Need to combine
− SIL
− Thermal rating
− Cost initial and life cycle
o (Taking into account corona, magnetic fields, mech loading etc)

33. FACTOR 1 Life Cycle Cost (k1)
33
• Covers determination of optimum aluminium area required. (Kelvin’s law)
• Cost of maintenance (estimate)
• Cost of losses – use system losses not line losses. (Due to power flow in
interconnected system)

34. FACTOR 2 THERMAL (k2)
34
• Cost is directly proportional to Thermal rating
− Higher rating higher initial cost
• A ratio is therefore needed
− Initial cost/MVA thermal (emergency or normal)
• The lower the ratio the better.

35. FACTOR 3 SIL (k3)
35
• The higher the SIL the higher the initial cost (normally)
• Ratio is therefore also required
− Initial cost/MVAsil

36. COMBINATION OF THE FACTORS
36
• Objective Matrix method
− Present practice is given 3/10
− 0 or 10 level is determined (normally trial and error) and a linear
interpolation used.
• ATI = w1k1+w2k2+w3k3
− wn are weighting factors

37. DETERMINING SCORE – NEG SLOPE
Ratio value
S
c
o
r
e
Ratio of current practice
Provides score of 3
3
10
(x1;y1)
(x2;y2)
Assume ratio
value that is ideal.
Score of ideal ratio is 10
Ratio of optionA
Score of
optionA
Y=mx+c
1. For the ratio (LCC, IC/MVA etc),
allocate a score of 3 for the
current practice. This gives point
x1;y1.
2. Assume an ideal ratio that will not
be exceeded. Allocate a score of
10 for this ratio value. This will
provide point x2; y2.
3. Calculate the straight line
equation using these two points.
4. Use the straight line equation to
determine the scores for the
different design options.
5. You now have dimensionless
scores which can be added.
CALCULATION OF SCORE FOR
SITUATIONS WHERE THE LOWER
THE RATIO THE BETTER THE
DESIGN

38. DETERMINING SCORE – POSITIVE SLOPE
Ratio value
S
c
o
r
e 3
10
x1;y1
x2;y2
y=mx+c
Ratio value of current practice
allocated score of 3
Ratio value of ideal design.
Allocated score of 10
Ratio value of optionA
Score of
optionA
1. Calculate the ratio of the current
design option. Allocate it a score of 3
2. Assume an ideal design value and
allocate it a score of 10
3. This provides two points. Calculate
the straight line (y=mx+c) graph
equation from points x1;y1 and x2;y2
4. Using the equation determine the
scores for the other design options.
5. This provides a dimensionless score
which can be added.
CALCULATION OF SCORES WHERE
THE HIGHER THE RATIO THE
BETTER THE DESIGN

39. CONDUCTOR SELECTION
CONDUCTOR OPTIMISATION
0.00
100.00
200.00
300.00
400.00
500.00
900.00
800.00
700.00
600.00
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400
Pmean(MVA)
LCC(Rmil)
2 X IEC800
4 X KINGBIRD
3 X BERSFORT
3 X TERN
3 X YEW
4 X TERN3 x Bersfort
39
4 x Tern
4 x Kingbird
3 x Tern

40. TOWER SELECTION
40

41. EXAMPLE LINE
41
− Quad “Zebra” guyed Vee tower
− Triple “Bunting” conductor guyed Vee tower
− Quad “Bunting” cross rope suspension (CRS) tower phase spacing of 6,5m.
− Quad “Rail” conductor with a CRS tower with a 6,5m phase spacing.
− Triple “Bittern” conductor with a CRS tower with a 6,5m phase spacing.
− Quad “Boblink” conductor with a CRS tower with a 6,5m phase spacing.
− Triple “Bersfort” conductor with a CRS tower with a 8,2m phase spacing.

42. ATI SCORES
42
CASE AL AREA
mm2
DESCRIPTION K1 (LCC) K2
(CI/MVAth)
K3
(CI/MVAsil)
1 1715 4XZEB V 103,53
[3,30]
28,13
[3,07]
7,43
[3,19]
2 1817 3XBUNT V 84,4
[6,25]
19.48
[5,20]
6,31
[5,38]
3 2423 4XBUNT
CRS 6.5m
88,36
[5,64]
13.27
[6,73]
7,02
[3,96]
4 1935 4XRAIL
CRS 6.5m
87,76
[5,73]
14.32
[6,47]
5,94
[6,12]
5 1933 3xBIT
CRS 6.5m
82,91
[6,48]
17.86
[5,60]
6,31
[5,38]
6 2901 4Xbob CRS
6.5m
93,33
[4,87]
17.04
[5,80
8,06
[1,88]
7 2059 3xBers CRS
8.2m
80,41
[6,81]
16.23
[6,00]
6,30
[5,40]

43. ATI WEIGHTING
43
CASE W1;W2;W3 W1;W2;W3 W1;W2;W3 W1;W2;W3
0,8;0,1;0,1 0,6;0,2;0,2 0,4;0,3;0,3 0,2;0,4;0,4
1 2,82 [7] 2,89 [7] 2,96 [7] 3,03 [7]
2 5,80 [3] 5,67 [4] 5,55 [4] 5,42 [4]
3 5,23 [5] 5,18 [5] 5,14 [5] 5,09 [5]
4 5,56 [4] 5,74 [3] 5,93 [2] 6,11 [1]
5 6,04 [2] 5,90 [2] 5,77 [3] 5,63 [3]
6 4,33 [6] 4,21 [6] 4,08 [6] 3,96 [6]
7 6,42 [1] 6,24 [1] 6,06 [1] 5,88 [2]

44. FINDINGS/BENEFITS
44
• Tower, foundation, hardware, electrical designers work together with planners
(iterative process)
• Indicator very sensitive and detects errors rapidly
• Line optimisation is possible looking at overall line design.
• Reliability is assumed constant for options
• Cost system is critical
• Most aspects of the line design are taken into account

45. CONCLUSIONS
45
• Line design options can be objectively determined
• ATI is a guide from which options can be finalised.
• Alignment with Planners requirements

46. GUIDE TO OVERALL
LINE DESIGN
GUIDE TO OVERHEAD OHL DESIGN TB 638
DC LINE DESIGN

47. DC LINE DESIGN
GUIDE TO OVERHEAD OHL DESIGN TB 638
ELECTRICAL CHARACTERISTICS

48. Power Transfer
x
Pmax
V 2
 vd%
100R L
where vd% is the voltage drop expressed as a % of V,
and the equation applies for both 10% and 15% volt drop. {1/sqrt(44) is an approximation of 15/100} and the format
removes the generality of the basic and simple equation.
V=Sending end voltage, pole to ground in kV
Rx=DC Resistance of the conductor in ohm/km
L=Distance in kilometres.

49. DC resistance
• DC resistance not dependent on current
• Dependent on conductor geometry and conductivity of material.
• Dependent on temperature.

50. Effect of conductor radius (TB388)
The higher the conductor diameter and the more sub-conductors in the bundle the more resistant the bundle is to
corona and therefore the designer is more able to raise the voltage to ground and hence increase the power capability
of the line.

51. Corona power loss
    





 225 3 3.05
 20log
 25 
 15log n  10log HS dE
P  11 40log mas
Where P is the corona loss is dB above 1 W/m, Emax is the positive polarity maximum bundle gradient in kV/cm, d is the
sub-conductor diameter in cm, n is the number of sub-conductors in the bundle, H is the average conductor height in m
and S is the pole spacing in m.

52. INSULATOR CONSIDERATIONS
• Similar requirements to AC as far as tower window design.
• For glass insulators need germanium glass as normal glass will shatter
− Zinc collar also required
• Creepage larger than for AC
• Space charge considerations as well as uneven pollution on insulator to be
taken into account.
• Composite insulators can be used for AC and DC lighter weight often suit
long insulator installation.
• Porcelain disc are also successfully used.

53. Summary of options
Action Parameter Voltage drop Corona Mechanical loading Thermal rating
+ and - pole spacing
decrease
Neutral Bad Good Neutral
Large Al area/cond (less
conductors)
Good Bad Good Bad
Diameter bundle
increase
Neutral Bad Bad Neutral
High steel content Neutral Neutral Bad Good

54. Voltage selection (TB 388)

55. DC LINE DESIGN
OBJECTIVE INDICATOR
GUIDE TO OVERHEAD OHL DESIGN TB 638

56. Optimisation process
• Select voltage (TB388)
• Determine range of conductor, bundle diameter and number of sub-conductors as
well as height above ground and pole spacings that will meet corona limitations
and power flow.
• Determine range of tower, foundation and conductor configurations.
• Finalise by further analysis the most suitable tower, foundation, conductor bundle
option. Recheck with power flow requirements, converter cost and technology.

57. Objective indicator
th
IC
3
MVA
ATIDC  w1LCC  w2IC* Posscoronal  w
ATIdc Appropriate Technology Index for DC lines
LCC is the life cycle cost expressed in terms of a score from 1 to 10 and IC is the initial cost.
Plosscorona is the power loss due tocorona.
IC is the initial cost.
MVAthermal is the thermal rating of the line and depends, as in the AC case, to the templating temperature of theline.

58. Line requirements
• DC voltage (V) 600;700;800 kV
• Number of sub-conductors per pole (N) 4; 5; 6
• Conductor type ACSR
• Line length 1750 km
• Transmitted Power 3000 MW (bipolar)
• Cost of the losses 60 U$/MWh; loss factor =0,5
• Life= 30 years; yearly interest rate= 10%
• Interest during construction 10%; maintenance 2% per year (of initial cost)

59. Alternate designs

60. Scores

61. ATIdc weighted scores

62. GUIDE TO OVERALL
LINE DESIGN
COST OF LINES
GUIDE TO OVERHEAD OHL DESIGN TB 638

63. Purpose
• The purpose of the questionnaire was to compare the work done by WG09
in 1990 to the latest figures as many component costs may have changed.
• The questionnaire was in two parts, the first to compare component costs
of existing projects, the second to compare costs of an actual line with
given parameters.
• The response was generally poor with around 13 respondents compared
with over 100 in 1990.

64. North America

65. Europe

66. Africa

67. Asia

68. Cost per km comparison

69. Competitive line costs
• From the graphs, if the conductor cost as a percentage of total line cost is greater
than 10% the cost is likely to be relatively low. The range of percentages in the
examples received indicated that the conductor cost for relatively low cost per km
lines, should vary between 10 and 15%.
• In the previous survey it was found that the conductor cost was 32% of the
material cost which was 63%. This results in 20% of the total line cost. It could
be concluded that the cost/km of lines has increased from 1990 to 2014 in real
terms mainly due to cost of labour and environmental issues.
• A good target for a competitive cost per km line would be that the conductor cost
should be between 15 and 20% in 2014.

70. COMPARISON TO 1991
YEAR
MATERIAL
COSTS
CONSTRUCTION
COSTS
CONDUCTORS SHIELD WIRE INSULATORS TOWERS FOUNDATIONS
For all lines and voltages 1991 63.7 36.3 32.7 3.8 8.1 36.2 19.2
2013 42.4 57.6 31.8 2.7 7.6 46.3 11.6
For all lines up to 150kV 1991 64.3 35.7 31.6 4.1 8.8 36.0 19.5
2013 46.4 53.6 28.6 2.0 7.9 49.6 11.9
For all lines over300kV 1991 62.6 37.4 34.1 3.9 6.9 36.4 18.7
2013 46.8 53.2 35.7 3.0 7.2 42.7 11.4
All single circuit lines 1991 63.6 36.4 33.1 4.2 8.2 35.6 18.8
2013 42.8 57.2 33.4 2.8 6.9 43.7 13.3
All double circuit lines 1991 63.8 36.2 32.0 3.3 7.9 37.1 19.7
2013 31.0 69.0 24.7 2.3 10.6 58.1 4.3
Guyed structure lines 1991 59.6 40.4 32.8 3.2 8.3 36.0 19.8
2013 55.0 45.0 36.5 3.2 6.3 41.3 12.7
Lines with 1 conductor/phase 1991 64.4 35.6 32.2 4.2 8.5 36.3 18.8
2013 38.7 61.3 28.3 2.0 7.8 45.1 16.9
Lines with 2 conductors/phase 1991 64.6 35.4 32.3 4.0 8.1 36.2 19.4
2013 38.0 62.0 32.3 2.3 10.6 48.4 6.3
Lines with 3 conductors/phase 1991 60.8 39.2 35.1 3.7 7.0 40.3 13.8
2013 41.5 58.5 36.6 4.6 6.6 42.6 9.6
Lines with 4 conductors/phase 1991 61.4 38.6 33.4 2.7 7.6 33.4 22.9
2013 56.5 43.5 34.2 3.4 7.9 37.9 16.7
Conductor, shield wire etc % as a function of material costs.

71. Percentage comparisons with 1991
YEAR
MATERIAL
COSTS
CONSTRUCTION
COSTS
COND
SHIELD
WIRE
INSUL TOWERS FNDN
For all lines and
voltages 1991 63.7 36.3 32.7 3.8 8.1 36.2 19.2
2013 42.4 57.6 31.8 2.7 7.6 46.3 11.6
For all lines up to
150kV 1991 64.3 35.7 31.6 4.1 8.8 36.0 19.5
2013 46.4 53.6 28.6 2.0 7.9 49.6 11.9
For all lines over
300kV 1991 62.6 37.4 34.1 3.9 6.9 36.4 18.7
2013 46.8 53.2 35.7 3.0 7.2 42.7 11.4
All single circuit
lines 1991 63.6 36.4 33.1 4.2 8.2 35.6 18.8
2013 42.8 57.2 33.4 2.8 6.9 43.7 13.3
All double circuit
lines 1991 63.8 36.2 32.0 3.3 7.9 37.1 19.7
2013 31.0 69.0 24.7 2.3 10.6 58.1 4.3
Guyed structure
lines 1991 59.6 40.4 32.8 3.2 8.3 36.0 19.8
2013 55.0 45.0 36.5 3.2 6.3 41.3 12.7
Lines with 1
conductor/phase 1991 64.4 35.6 32.2 4.2 8.5 36.3 18.8
2013 38.7 61.3 28.3 2.0 7.8 45.1 16.9
Lines with 2
conductors/phase 1991 64.6 35.4 32.3 4.0 8.1 36.2 19.4
2013 38.0 62.0 32.3 2.3 10.6 48.4 6.3
Lines with 3
conductors/phase 1991 60.8 39.2 35.1 3.7 7.0 40.3 13.8
2013 41.5 58.5 36.6 4.6 6.6 42.6 9.6
Lines with 4
conductors/phase 1991 61.4 38.6 33.4 2.7 7.6 33.4 22.9
2013 56.5 43.5 34.2 3.4 7.9 37.9 16.7

72. Summary of Trends
• Material and construction costs – the trend appears to be that the material cost has reduced as a function of
total cost with the construction cost being the more prevalent cost. This appears to be the case over the entire
range of lines investigated.
• Conductors – in the 2013 cases the steel shield wire is included in the conductor cost. Even with this inclusion,
it appears the conductor cost is generally the same or lower percentage of the total cost as compared to 1991.
• Shield wire – this cost is related to the OPGW cost for 2013 and the steel wire cost for 1991. The sample for
double circuit and single circuit lines for 2013 is very small and therefore cannot be considered to be
representative. However it indicates a similar percentage to the 1991 costs even though the shield wire is more
complicated and expensive in real terms in 2013.
• Insulators – the percentage of total cost spent on insulators seem to be slightly lower than in 1991. This could
be due to the advent of composite insulators which have dropped in price considerably over the years as well as
glass being more competitive with merger of manufacturers.
• Towers – The percentage of the total cost spent on towers seem to be higher than in 1991. This cost includes
the erection cost which could indicate the higher cost of labour which is reflected in the construction cost
compared to material cost. As mentioned previously the environmental constraints on current lines could have
resulted in more angle or strain towers as well as more aesthetically pleasing towers such as the Wintrack
towers.
• Foundations – The percentage of the total cost spent on foundations seems to be lower than is 1991.
This may be due to the higher level of mechanisation and perhaps use of more pile foundations
but this is not confirmed.

73. GUIDE TO OVERALL
LINE DESIGN
EXAMPLES OF OPTIMISATION
GUIDE TO OVERHEAD OHL DESIGN TB 638

74. Tower development
Proposed CRS 6%
saving on line cost
74
Existing guyed V

75. Cost Savings
750-15 degree structures 15-30 degree structures
R0
R75,000
R150,000
R225,000
R300,000
R450,000
R375,000
Misc Costs
Insulation
Hardware
Tower Erection
Tower Supply
Foundations
52%
Saving
46%
Saving

76. 705A tower at NETFA
76

77. REFERENCES/
ACKNOWLEDGEMENTS
77
[Stephen 2004] Stephen R. “Use of indicators to optimise design of overhead transmission lines”. Paper330-1
Shanghai Symposium, Cigré 2003. (Held in Lubljana April 4-62004)
[Stephen 2011] Stephen R “Objective detetermination of Optimal power line designs” PhD thesis submitted in 2011
University of Cape Town.
[Muftic]. Muftic D, Bisnath S, Britten A, Cretchley DH, Pillay T,Vajeth R “The Planning design and constructionof
overhead power lines” Published by Crown publications 2005 ISBN 9780620330428
[Southwire] Overhead Conductor Manual First edition copyright1994.
Prof. C.T.Gaunt (UCT) acknowledged for comments andinput.
J. Lindquist AC/DC conversion TB583
Nolasco, Jardini, TB 388