conductor creep 2

[DLK]

Hi,

Could any one send me name of some reference book, standard etc. or way of calculation for conductor creep and resultant temperature shift.

I need basic idea of applying the temperature shift to the sag-tension calculation of conductor.

Thanks

 

[darron]

A maunual we use a manual HB C(b)1, guidelines for the design and maintenance of Overhead Distribution and Transmission lines.

This is an manual produced by the Electricty Supply Association of Australia Ltd.

Hope this is some help

 

[gavaskar]

Hi, I'm Dilip Nale. The answer to your question is as below.

1. Regarding references, the procedure is given in one of the CIGRE Journal. I dont remember the exact no.

2. As per Indian code IS 802, a simple approch is stated.
Usualy at every day temp still air condition, the conductor tension allowed is 25%. With this as starting condition you can generate sag tension calculations and find out design tensions and max sag.

For creep calculation, find backward the temperature at which conductor tension does not exceed 35%. DEfinately this temperature will be less than every day temperature.
The difference between this calculated temperature and design everyday temperature is to be considered as tempearture shift to be used for preparation of INITIAL stringing charts.
If the towers are strung wirh these initial tension ( With reduced temperature) will result into final design tension after say 10 years. ( Dont worry. Creep is very fast during first 72 hours)

Hope this clarifies your query. If you need further clarification pl write to me .

Regards,

D.N.Nale

 

[ergon]

Basically there are three sources of strain. Thermal, elastic and plastic (metallurgical creep and strand settling). It is a common practice to convert long term creep into an equivalent value of thermal strain (called the creep compensation temperature). When sagging the conductor, the creep compensation temperature is subtracted from the actual conductor temperature and the sag is read from the final sag table.

The formula is T = (Ef - Ei)/a
where T = temperature compensation (C or F);
Ef = final creep strain at say 10 years;
Ei = initial creep strain at time of sagging;
a = coefficient of thermal expansion (/C or /F).

Creep strain is a function of conductor stress (tension), conductor temperature and duration. You should be able to obtain coefficients for a creep prediction equation from a conductor manufacturer. However because the conductor tension reduces as the conductor ages, calculating the creep with a time step of 10 years will result in a larger value of creep than say ten 1 year time steps.

Note also that additional strand settling will occur if the conductor experiences a heavy load during its lifetime.

 

[rollingburnout]

IEEE proceedings

1979

"TECHNIQUE TO INCLUDE ELEVATED TEMPERATURE CREEP IN CONDUCTOR SAG-TENSION CALCULATIONS"

Authors Harvey and Larson

I was unable to reproduce the example problems I am sure I am missing something simple, If you figure it out let me know.

 

[jalexiou]

Typically conductors experience only primary creep and thus obey the power law of, strain = C*time^m for a constant stress & temperature level. Here C, m are creep coefficients. In general, C depends on both temperature and the stress level with a similar equation C=K*T^n*(stress/uts)^k, where K,n,k are also coefficients, T is the temperature in absolute degrees, and uts is the ultimate tensile strenght of the cable. Typically m=0.16, although it varies between m=0.13..0.19. The other coefficients I don't remember.

In real life the tension & temperature changes through out time. Typically creep is considered at the everyday temperature but with steps of stress levels as mentioned at above posting. In addition, there might be Elevated Temperature Creep, where the cable is operated for a few hours at high temperatures for additional creep but this only applies to cables with a high content of aluminum where the stretched aluminum may reach a compressive state.



John Alexiou
Alcoa - SAG10/Vibrec Development.