Reliability Based Design of Transmission Line

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Reliability based design of

Transmission Line


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Design philosophy

Reliability

Security

Safety


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Types of towers Suspension towers

Tangent towers (0o) with suspension string

for straight runs only.Intermediate towers (0o to 2o) with

suspension string for straight runs andupto 2o line deviation.

Light angle towers (0o to 5o) withsuspension string on straight runs andupto 5o line deviation


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4. Dead end towers with tension string for use as dead end ( terminal) tower oranchor tower.

5. Large angle and dead end towers withtension string for line deviation from30o to 60o or for dead ends.


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Reliability levels Return period Reliability level

50 years 1

150 years 2

500 years 3


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Wind effects

BasicW

ind Speed Meteorological Reference Wind Speed

Design Wind Speed

Risk Coefficient K1 Terrain Roughness coefficient K2


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Terrain categories1. Exposed open terrain seacoasts,

open stretch of water, deserts and flattreeless plains

2. Open terrain - normal country lineswith very few obstacles.

3. Terrain with numerous obstructions built up areas and forest areas


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Design Wind Pressure

Design wind pressure (N/m2) = 0.6 Vd2

Where Vd = design wind speed in m/s


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Wind loads

Wind load on tower

Wind load on conductor and ground

wire

Wind load on insulator strings


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Temperature variations Maximum and minimum temperatures

as per isopleths

Average everyday temperature shall be32o C

Maximum conductor temperature for

design 75o for ACSR

85o for AAAC


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Sag and TensionAs per IS 5613( Part 2 / Sec 1) : 1985

100%Design Wind Pressure atEveryday Temperature

36%Design Wind Pressure at minimumtemperature


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Loads on tower Climatic loads

Failure containment loads

Construction and maintenance loads


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Computation of loads Transverse loads

Reliability requirements

Security requirements

Safety requirements


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Vertical loads



Safety requirements


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Longitudinal loads



Safety requirements


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Loading combinations Reliability condition

Transverse loads

Vertical loads

Longitudinal loads


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Loading combinations Security condition

Transverse loads

Vertical loads

Longitudinal loads


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Loading combinations Safety condition

Transverse loads

Vertical loads

Longitudinal loads


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Anti cascading checksAll angle towers shall be checked for

the following anti cascading conditions

with all conductors and ground wireintact only on one side of the tower

Transverse loadsVertical loads

Longitudinal loads


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Tension limits Conductor / Ground wire tension at

everyday temperature and without

external load should not exceed thefollowing percentages of UltimateTensile Strength

Initial unloaded tension 35%

Final unloaded tension 25%


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The ultimate tension under everydaytemperature and 100% design wind

pressure Or

Minimum temperature and 36% design

wind pressureShall not exceed 70% of Ultimate TensileStrength of the Conductor / Ground wire


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Broken wire conditions

Single circuit towers

Any one phase or ground wire brokenwhichever is more stringent for a particular

member


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Double , Triple and Quadruple circuit towers

Suspension towers

Any one phase or ground wire broken whicheveris more stringent for a particular member


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Small and medium angle towers

Any two phases broken on the sameside and same span or any one phaseand one ground wire broken on the

same side and same span whichever ismore stringent for a particular member


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Large angle tension towers /Dead endtowers

Any three phases broken on the same sideand same span or any two of the phases andone ground wire broken on the same sideand same span, whichever is more stringentfor a particular member


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Reliability Based Design of Transmission Line

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