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OPTIMAL LINE DESIGNS

R. StephenESKOM

PRESENTATION TO SC B2 ICELAND 2011

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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.

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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

VLL is phase to phase

voltage

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Determination of R, X and B

• Resistance is a function of– Conductor construction material and line length

• Lay ratio, ACSR, AAAC, number of layers, diameter of strands.

– Temperature• The higher the temperature the higher the resistance

– Current and frequency• Transformer effect• Eddy currents.

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Variation with current

Penguin(1+6)Tcond=80°C

1.00

1.02

1.04

1.06

1.08

1.10

1.12

1.14

0 100 200 300 400 500 600 700 800I(A)

Rac/

Rdc

Falcon(19+54)Tcond=80°C

1.00

1.02

1.04

1.06

1.08

1.10

0 1000 2000 3000 4000 5000 6000 7000 8000 I(A)

Rac

/Rdc

107 mm2

800 mm2

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Determination of L

• L is a function of GMR and GMD

• Larger bundle radius and closer phase spacing gives lower L

r’=0.7788xradius of conductor (m)

Rb=radius of bundle n=number of subconductors

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GMD EXAMPLES

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Determination of C

• To increase capacitance keep phases closer together.

• P=1/C

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LINE MODEL

• X = jωL• B=jωC/2• R=R

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Summary

• 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

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Corona limitations

• Corona (often producing audible noise) is a function (inter alia) of phase spacing and bundle size– Smaller bundle radius better– Wider phase spacing better– More sub conductor bundles better.

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Audible noise as a function of voltage

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Mechanical considerations

• Wind load is major consideration in tower design– Less conductors in the bundle the better– Less UTS the better (Lighter strain towers). Higher tension to

increase height.

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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 OHS

act– Height determined by voltage and flashover distance

• Heat Balance equation used.

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Joule and magnetic heating

• Joule dependent on AC resistance and temperature• Magnetic heating dependent on current and conductor

layers.

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Solar heating

• Darkness of conductor• Diameter of conductor• Solar radiation

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Convective cooling

• Dependent on – the conductor diameter (bigger is better)– Wind speed– Temperature difference (bigger is better)– Roughness

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Templating temperature

• Templating temperature is the conductor temperature at which the height above ground is in accordance with the OHS act

ConductorTemplating temperature deg C

Normal Amps

Emergency Amps

TERN 50 611 814TERN 60 784 991TERN 70 911 1138TERN 80 1023 1257ZEBRA 50 642 859ZEBRA 60 818 1049ZEBRA 70 963 1203ZEBRA 80 1080 1325

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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

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Planning requirements

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

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Conductor size and temp

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Material considerations

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Material - core

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Conductor type

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Selection of conductor

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Indicator to determine best design

• Need to combine – SIL– Thermal rating– Cost initial and life cycle

• (Taking into account corona, magnetic fields, mech loading etc)

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FACTOR 1 Life Cycle Cost (k1)

• 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)

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FACTOR 2 THERMAL (k2)

• 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.

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FACTOR 3 SIL (k3)

• The higher the SIL the higher the initial cost (normally)• Ratio is therefore also required

– Initial cost/MVA sil

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COMBINATION OF THE FACTORS

• 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

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CONDUCTOR SELECTION

CONDUCTOR OPTIMISATION

0.00

100.00

200.00

300.00

400.00

500.00

600.00

700.00

800.00

900.00

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400

Pmean(MVA)

LCC

(Rm

il)

2 X IEC8004 X KINGBIRD3 X BERSFORT3 X TERN3 X YEW4 X TERN3 x Bersfort

4 x Tern

4 x Kingbird

3 x Tern

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TOWER SELECTION

+ +

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SERVITUDE AND

suspensionCross- Rope

+

+

400kV

8.5m

Min.Conducto rclearance

Servitude

21.0m55.0m

28.0V V

VV

VV

V

V

36.0m(average)

+ +

+

+

MinimumConducto rclearance

+

+ +

+

+ + +

+

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+ + +

8.5m

V

VV

V

VV

+ + + +

+++

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SERVITUDE AND400kV

Guyed suspensiontype

26.0m

Servitude

55.0m V

33.0m(average)

23.0mV V

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Tower development

Proposed CRS 6% saving on line cost

Existing guyed V

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Cost Savings

0-15 degree structures 15-30 degree structures

R0

R75,000

R150,000

R225,000

R300,000

R375,000

R450,000

Misc CostsInsulationHardwareTower ErectionTower SupplyFoundations

52% Saving

46% Saving

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Performance comparison

Improved performance can give

0.05-0.1faults/100km/annum

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705A tower at NETFA

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EXAMPLE LINE

– .Quad “Zebra” guyed Vee tower– .Triple “Bunting” conductor guyed Vee tower– .Quad “Bunting” cross rope suspension (CRS) tower with a 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.

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ATI SCORES

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]

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ATI WEIGHTING

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]

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FINDINGS/BENEFITS

• 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

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Double Circuit developments

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Comparison self supporting vs CRS

Bulk power transfer capacity 5,000 MWMax tower height (Self Support) 77,5 mMax tower height (Cross Rope) 53,2 mPerformance (Faults / 100 km / year) < 0,3Visually acceptable YesEfficient land use 57 %Conductor Bundle 8 x BersfortMax Altitude (AMSL) 1,650 mCountry wide application Yes

Self Support Tower (Figure 3a) Electric field (max) 9,7 kV /m Audible Noise 43,5 dBA Radio Interference 48,3 dBμV/m Magnetic Field (@ 523 A) 3,9 μT SIL 2,581 MW

Cross Rope Tower (Figure 3b) Electric field (max) 10 kV/m Audible Noise 49,8 dBA Radio Interference 56,3 dBμV/m Magnetic Field (@ 523 A) 4,3 μT SIL 2,904 MW

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CONCLUSIONS

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

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REFERENCES/ACKNOWLEDGEMENTS

• [Stephen 2004] Stephen R. “Use of indicators to optimise design of overhead transmission lines”. Paper 330-1 Shanghai Symposium, Cigré 2003. (Held in Lubljana April 4-6 2004)

• [Muftic]. Muftic D, Bisnath S, Britten A, Cretchley DH, Pillay T, Vajeth R “The Planning design and construction of overhead power lines” Published by Crown publications 2005 ISBN 9780620330428

• [Southwire] Overhead Conductor Manual First edition copyright 1994.

• Prof. C.T.Gaunt (UCT) acknowledged for comments and input.