Forces on Conductors

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Special Forces Appendix C

Snow and Ice Loading Appendix EE

Robert Lake

Senior Development Engineer

Transpower NZ Ltd.

Special Forces

Forces due to Short-Circuit

Currents;

Creeping Snow (on

foundations/lower part of

structure);

Earthquakes; and,

Mining Subsidence.

Australian Panel B2 Overhead Line

Design Seminar AS/NZS 7000:2010

Overhead Line Design Sydney 28 & 29

March 2011


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Forces due to Short-Circuit Currents Different for simplex or bundled phases;

IEC60865-1 provides simplified method;

Based on initial/everyday tension in system;

Terminal or landing spans, short strain spans

critical, but may affect several spans from

substation;

Produces increased conductor tensionswhich may be greater than wind or other

loading induced tensions.




March 2011

Forces due to Short-Circuit Currents

Only 2-phase short-circuit current considered;

Pinch tensile force from sub-conductor

attraction;

Swing tensile force due to phase to phase

attraction;

Vertical drop tensile force due to phase to

phase attraction;

IEC60865-1 provides some example

calculations.




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Forces due to Short-Circuit Currents

Wn Coincident wind pressure of 25%

ultimate;

Ft tensions for non short-circuit spans;

Fsc* maximum tension from 3 cases in

IEC60865-1;

1.25 factor for exceptional load; Loading considered to be too quick to affect

foundation significantly.




March 2011

Creeping Snow

Loading will probably be

different in all situations;

May involve taller pedestal

outstands on foundations

designed to accommodate

lateral loading.




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Earthquakes Very topical;

Generally not a major issue with loading for

transmission lines;

Location relative to fault line is important;

Distribution poles with mounted transformers

can involve significant impact;

AS1170.4-2007 Australian actions; and, NZS1170.5-2004 New Zealand actions.




March 2011

Earthquakes Need to allow for potential settlement,

rotation and/or translation movements;

Seismic mass to include all permanent

attachments; Fundamental period;

Ductility factor by material/structure type;

Cables/conductor behaviour linear springs.




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Earthquakes Types of analysis:

Equivalent static force method symmetrical and

consistent stiffness and mass, relative low

structure height;

Modal response spectrum analysis significant

mass or stiffness variations; and,

Time history analysis method large guyed

structures.

Combination of effects multiple directions;

Second order effects P- effects.




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Earthquakes




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Earthquakes




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Earthquakes




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Earthquakes




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Earthquakes




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Earthquakes




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Earthquakes




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Mining Subsidence Areas subject to underground mining, close to

large excavations of weaker material, or de-

watering activities e.g. geothermal activity

Likely to be more significant for towers than for

poles;

Tie-beams can be used between all four legs

to minimise differential settlement;

Loading will depend on potential for differential

settlement, extent of settlement etc.




March 2011

Snow and Ice Loading

With or without coincidentwind;

Variation with altitude/location;

Australian requirements: Ice above 800m;

Ice above 500m in Tasmania.

New Zealand requirements: Snow and Ice varies widely.

Guidelines only. Use local

knowledge e.g. BoM ifavailable.




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Snow and Ice Loading Australia generally:

12mm, 900kg/m3, 100Pa wind, -5 C;

Assumed 50yr return period;

Unbalanced loading should be

considered.

Tasmania:

All areas;

Structure ice area;

Defined wind Pa; Apply 100m lower

in some areas.




March 2011


New Zealand:

Based on 50yr return period loading;

Uses a reliability multiplier instead of varying return

period as used for wind;

Allows to apply higher

factor for special assets;

Snow and ice loads from

Table EE3 multiplied by Mrel;

Conductor temperature under snow 0C;

Conductor temperature under ice varies with altitude.




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Snow and Ice Loading New Zealand:

As per IEC60826,

unbalanced tensions based

on 70% of full loading on one

side and 30% on other side;

Large deviations/

terminations suggest full ice

on one side only;

Care with post or pininsulators which limit flexibility

of system.




March 2011


New Zealand:

Wind load from

AS/NZS1170.2 for

relevant return

period;

Only SW, S, or SE

wind directions;

Snow 400kg/m3;

Ice 700kg/m3;

Differential snow/ice

for torsion and longl.




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Special Forces Appendix C

Snow and Ice Loading Appendix EE

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Forces on Conductors

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